This invention is relates to method of the dry etching and an apparatus for use in such method. More particularly, the invention relates to method of selective dry etching for multi-layered structures.
In recent years, the degree of integration of semiconductor integrated circuits has advanced, and circuit pattern sizes also have become finer. Thicknesses of various thin films utilized in the fabrication processes for semiconductor integrated circuits have become very small with the reduction in size of the circuit patterns. For instance, the thickness of a gate oxide film of a MOS type integrated circuit presently may be as small as 100 .ANG..
The reactive ion etching process is well known as one method of etching electrode materials formed of polycrystalline silicon, etc. The reactive ion etching process ordinarily includes the following steps. An object to be etched is disposed between a pair of parallel plate electrodes provided in a vacuum chamber and a reactive gas is introduced therein. Thereafter, the reactive gas is caused to discharge by applying radio frequency power. As a result, gas plasma generates from the discharge of the reactive gas. The object is etched by the gas plasma.
Plasma etching, ECR type dry etching, ion beam etching and photo excited etching, etc., in addition to reactive ion etching, are known methods for etching. These etching processes are also performed by operating chemically or physically on the object with ion of activated reactive gas.
Reactive ion etching is largely classified into two types. One is the cathode coupled type in which the object is disposed on the electrode applied with the radio frequency power. The other method is the anode coupled type in which the object to be etched is disposed on a grounded electrode. The electrode with object thereon is usually water-cooled to a normal temperature to prevent thermal degradation of a photo-resist formed on the surface of the object. The object is chucked electrostatically or mechanically, or merely placed on the water-cooled electrode.
In all the above-mentioned radio frequency coupled types, there is an ion assisted chemical reaction to carry out the etching. Ions existing in the plasma bombard the object and the chemical reaction carries out the etching naturally using an active radical of the reactive gas. The ion assisted chemical reaction is the most suitable for anisotropic etching, and the chemical reaction is the most suitable for isotropic etching. The etching direction is more satisfactory and the shape of the etched wall is closer to the vertical, the more the contribution of the ion assisted chemical reaction.
When the adhesion between the object and object holder is strong, this is sufficient to water-cool the object holder to the normal temperature and prevent the deterioration of the photo-resist formed on the surface of the object. If the adhesion is not strong, the object is not cooled sufficiently to suppress the deterioration of the photo-resist, because the etching rate drops when the temperature of the object drops.
Problems of conventional etching apparatus will be explained below. FIG. 11 is a sectional view of a parallel plate type dry etching apparatus of the cathode coupled type. There is a pair of parallel plate electrodes (including an anode 2 and a cathode 3) within a vacuum chamber 1. The anode 2 is grounded, and the cathode 3 is supplied with radio frequency power of 13.56 MHz through a matching box 4. The cathode includes a cooling path 5 to supply cooling water as a coolant, and the cathode is thereby cooled. Etching gases are introduced from a gas flow tube 6 into the vacuum chamber 1, and are exhausted through an exhaust 7. A substrate 8 is disposed on the cathode 3.
In this conventional etching apparatus, the cathode forms a part of the vacuum chamber. The coolant of normal temperature flows into the cathode. However, water vapor liquifies and congelation forms on surface of the cathode 3 and the cooling path 5 based on the temperature difference between the inner temperature of the vacuum chamber 1 and the temperature of the cathode 3. As a result, water drips inside the apparatus, and short circuits occur.
For instance, a matching box 4 connected to the cathode 3 for adjusting impedance of the electrodes and the electric power is provided in the conventional etching apparatus. The matching box is provided under the electrode supplied with the radio frequency power. The congelation causes an electrical short in the matching box. A rubber O-ring, etc., is used to vacuum-seal the cathode side. This is generally heat-resisting. However, the rubber O-ring has hardens with the drop of the temperature of the cathode, and leakage occurs. There are similar problems in other conventional etching apparatus.
Problems caused in the etching of polycrystalline silicon using the conventional etching apparatus will be explained below. FIG. 12 shows the potential distribution in a reactive ion etching apparatus. Reference numbers 3 and 2 of FIG. 12 are the cathode and anode of the dry etching apparatus, respectively. The highest electric potential into the discharge space in the vacuum chamber is the plasma potential 10 shown in FIG. 12. Electrons are stored on all surface contacted with the plasma, so that the electron transfer is very great in comparison with the ions. As a result, the electric potential becomes lower than the plasma potential 10.
A big drop of the cathode voltage occurs near the surface of the cathode 3 to maintain the discharge, but the difference of the potential reaches only the plasma potential 10 near the surface of the anode 2. Therefore, the cathode coupled type has a good etching direction to contribute to the ion assisted chemical reaction. The anode coupled type has a low ion bombardment energy. Thus, the etching direction is not as accurate as the cathode coupled type. As a result, an undercut or inversely tapered feature, as shown in FIG. 2 (c), is apt to occur. Accordingly, the cathode coupled type is better from the view point of working efficiency, and this types is more suitable as a future fine pattern forming method of the submicron order.
A selectivity of the over material to the under material is important in addition to the working shape. For example, in an etching process of the gate material of polycrystalline silicon, etc., when the thickness of the gate oxide film becomes 100.ANG.or less, a very high selectivity is required. In the cathode coupled type, the surface is dissolved or activated independently of the character of the material, because the ion bombardment energy is large. As a result, the selectivity of the cathode coupled type is generally smaller than that of the anode coupled type.
There are problems in that the selectivity is good but the working efficiency is not good in the anode coupled type, and the working efficiency is good but the selectivity is not good in the cathode coupled type. When a regular MOS transistor is manufactured by the anode coupled type method, the working efficiency of the anode coupled type causes the scattering of the channel length. On other hand, in the cathode coupled type, etching does not stop on the gate oxide film and progresses to the under silicon substrate. This causes deterioration of the yield.
In recent years, an ECR (Electron Cyclotron Resonance) discharge method has been developed and applied to the etching of polycrystalline silicon. An selectivity of 30 or more is obtained, so that the ion energy is multiplier greatly. However, the working efficiency is lower than the cathode coupled type so that the ion energy is small, similar to the anode coupled type ion etching apparatus.